The view source button allows you to view the HTML source code for the body of the page you are editing. This feature is typically for users with a more advanced knowledge of HTML. There may be some more advanced web design techniques that you are not able to do within the WYSIWYG page editor and would require the ability to work directly with the HTML. The view source feature allows you to do this.
You can learn more about the view source feature and find information about html and scripts in the "HTML and Scripts" section of the help pages.
*Note: The view source feature gives the webmaster access to the HTML source code for the body of the page. This feature does not give the webmaster access to the source code for the template elements of the page.